Steady-state for an unperturbed disc initially confined to a narrow birth ring between 45 and 53 AU. a) Normalized surface density profile viewed head-on. b) Azimuthally averaged radial distribution of the geometrical cross-section σ as a function of particle sizes (as parameterized by their β value). At each radial distance, the σ distribution has been normalized to 1. The dotted line shows, for a given radial distance r, the β value of particles whose periastron is located in the middle of the ring and whose apastron is located at r.